From EP 0 321 809 (US equivalent U.S. Pat. No. 4,932,861), EP 0 780 629 (US equivalent U.S. Pat. No. 5,735,687), WO 93/17279 (US equivalent U.S. Pat. No. 5,402,633), and EP 0 945 677 (US equivalent U.S. Pat. No. 6,178,752), premixing burners are known in which a combustion air flow is introduced tangentially into a burner interior by means of a swirl generator and is mixed with fuel. At the burner outlet, the resulting vortex flow bursts open at a jump in cross section, inducing a recirculation zone which serves to stabilize the flame in operation of the burner.
The axial position of the recirculation zone which arises is of critical importance for the stabilization of the flame, and in its turn is substantially determined by the axial flow in the center of the burner. If this axial flow is too weak, the recirculation zone, and with it the flame, migrates into the burner interior. The danger then exists of a flashback of the flame and a gradual overheating of the burner. If, on the other hand, the axial flow is too strong, the recirculation zone can detach from the burner outlet and become unstable. The consequence can be strong, damaging, combustion pulsations or even an extinction of the flame.
Summarizing, the axial flow in the center of a burner of the aforementioned kind is thus of great importance for stable and safe operation. It is therefore also known to produce a defined axial central flow in such burners by means of a central air injection. Nevertheless, a more or less favorable position of the recirculation zone results even in these burners in different states of operation. Thus, at full load, an axial flow is desirable which is strong enough to hold the flame safely outside the burner. In contrast, at lower loading of the burner the axial flow has to be prevented from driving the recirculation zone impermissibly far from the burner mouth; the axial impulse of the central flow thus has to be smaller.
Solutions known from the state of the art are not capable of setting an optimum axial position of the recirculation zone under all operating conditions.